I have been having this problem as well, but only very recently, it is
making Pan almost unusable!

Some history... since I have been using Pan for a looong time one of
the first things I do when I install a distro is install Pan. When
Hardy came out I switched from Fedora to Hardy and all was good, never
had any speed problems except when loading huge lists and running out
of memory. When Intrepid came out I duly upgraded and Pan slowed to a
hopeless crawl. For that and several other reasons I re-installed
Hardy but the Pan problem persists.

Having read the earlier threads I have tried compiling using various
versions of GCC with no effect.

The fact that the version of Pan I used to have was fine but the clean
install I did a couple of weeks ago wasn't suggests a new package has
been created using more more recent libraries and there is some
incompatibility in the libraries.

Further, the problem seems to be related to header display. I have a
rule set up that allows me to display only the last couple of days
headers (please bring back the Display New Headers filter!) this often
results in a fairly small displayed list which can then be
manipulated, filtered etc reasonably quickly. As soon as that rule is
turned off it grinds to halt as Pan tries to display the much larger
list.